WO1999053615A2

WO1999053615A2 - A device and method for reduction of transients

Info

Publication number: WO1999053615A2
Application number: PCT/SE1999/000531
Authority: WO
Inventors: Anders GINGSJÖ; Bo HÅKANSSON
Original assignee: Gingsjoe Anders; Haakansson Bo
Priority date: 1998-04-08
Filing date: 1999-03-31
Publication date: 1999-10-21
Also published as: WO1999053615A3; SE9801235L; SE9801235D0; AU4065799A

Abstract

Device and method for reduction of signal transients, where the signal is divided into frequency bands, the device at least comprising a transient detector (1), a calculating means, means for delaying a signal (2) and an adaptive filter. In the device an input signal (4) is arranged to both the means for delaying a signal (2) and the transient detector (1) via parallel connection, which both are connected to the adaptive filter (3) the output (5) of which is transient reduced. Characterizing is that the detector (1) detects frequency bands with a threshold function, which indicate presence of transient, and it indicates presence of transient if derivative and/or input signal (4) level exceeds predetermined levels. The detector (1) activates, at the indication of a transient in a frequency band, a calculating means for calculation of a set of filter parameters, describing the gain of the frequency bands, for controlling the transient reduction. If the detector (1) detects presence of transient in only one frequency band, the filter parameters accomplishes a total gain to a level determined by the level of the input signal (4). When the detector (1) no longer detects presence of transient the device goes back to the previous set of filter parameters.

Description

T ITLE Device and method for reduction of transients

TECHNICAL FIELD The present invention relates to a device and method for reduction of transients to be used in connection to hearing aids, telephony, cellular telephony, transmission equipment, telephone switches, ear protection equipment or the like.

BACKGROUND OF THE INVENTION

For a person with impaired hearing and who uses a hearing aid transients are often perceived strongly negatively. It has been shown that persons with impaired hearing do not have easier to cope with sounds of high intensity than people with normal hearing, which leads to that one cannot just increase the sound amplification so that the person will hear the sounds with low intensities. This means that one tries to amplify sounds only so much as to avoid coming over the intensity level which the person perceive as troublesome. Also in the field of telephony there are problems in connection to transients. For persons with normal hearing transients in connection to telephony can be perceived negatively. One of the major problem in the context of transients is that these normally are amplified together with the sound one wishes to amplify, which can lead to very high intensities. One example of another problem area is persons using headsets in e.g. work, such as people working at telephone switch boards. A facsimile signal can easily damage the hearing of a telephone operator .

The problem with transient sounds, such as clatter, rattle and such, is that the energy often is concentrated to frequencies coinciding with the frequency band where the most acoustical amplification is needed in order to give the hearing impaired person en optimum apprehension of the speech. A transient is defined as a sound with a duration shorter than one second. If one give priority to apprehension of speech in a traditional hearing aid this will often result in the setting of the hearing aid which makes it unpleasant to use in the every day sound ambience .

Available methods to deal with this problem includes everything from equipping the hearing aid with cutting circuitry with variable activation levels or different kinds of automatic volume control. In spite of the problem of transients being well known, about 50% of total sales volume of hearing aids presently consists of hearing aids without any real protection against high transient noise or with simple cutting circuitry. Cutting circuitry results in poor sound quality and thereby a lower acceptance of the hearing aid. Different forms of automatic volume control (AVC) is used diligently in the remaining group of devices.

Automatic volume controls are divided into input controlled or output controlled ones, realized either by forward control or feedback control principle. Input control and output control refer to the location of the detector, the output of which controls the amplification. An input controlled AVC uses the input signal to the detector in a position before potential manual amplification control. Typically, the input controlled AVC is used to dynamically adjust the amplification of the hearing aid to the corresponding hearing increase of the hearing impaired person (compensation of poor sensitivity for silent sounds) . By adding a further AVC in the final amplifier it is possible to assure against too loud sounds being generated by hearing telephone if a too high amplification has been manually set, at the same time as a strong sound occurs. Important parameters in this context are the time constants, compression threshold and compression relation of respective AVCs . Hereby the following problem occur: In US-4,718,099 and US-5,144,675 are inventions disclosed which deal with this situation. In order to avoid distorting the sound the time constant of the input controlled AVC should not be too fast, and in order to avoid too low amplification after a sudden strong sound the time constant should not be too long. This problem has partially been solved by using so called adaptive compression or double detectors with different time constants that is short time constant for the release time for a transient situation and long time constant for speech having slowly varying signal strength. The drawback with all these AVC methods are:

Only limited damping of unpleasant transient sounds is achieved.

• Several fast transients lower the amplification in the hearing aid which can result in the bearer does not hear the succeeding speech signal.

• The attack time of the detector admits that a part of the initially very high sound levels are allowed to pass through the hearing aid with full amplification.

Realisation of the AVC in forward controlled state or feedback state differs principally in the way that the detector in a feedback controlled AVC (primarily analogue technique) can be made simpler since it can be constructed for handling a more limited dynamic range. A functional difference is however that with a forward controlled AVC it is possible to compensate the effect of time constants in the detector by using a delay in the signal path.

THE TECHNICAL PROBLEM

The technical problem dealt with by the present invention is reduction of transients in signals in a communication context, such as hearing aids, telephony, cellular telephony, transmission equipment, ear protection equipment or persons using headsets during work, such as telephone switchboard personnel. This problem has been solved by means of a device according to the present invention according to the indicated features in the patent claims.

THE MOST CHARACTERISING FEATURES OF THE INVENTION

The most characterising feature of the present invention lies in a method and device for reduction of transients in signals, where the signal is divided into one or more frequency bands, the device at least comprising a transient detector, a calculating means, means for delaying a signal and an adaptive filter in which device an input signal is arranged to both the means for delaying a signal and the transient detector both connected to the adaptive filter, the output of which is transient reduced. Characteristic features are -that the detector is arranged to detect at least one frequency band for the input signal (4) regarding presence of transient with at least one threshold function for indicating presence of transient in at least one frequency band, -that the detector is arranged to indicate presence of transient if an estimated derivative and/or energy or amplitude level for the detector input signal exceeds predetermined levels, -that the detector, when indicating transient in at least one frequency band, is arranged to activate a calculating means for calculation of a set of filter parameters, describing the gain of the frequency bands, for controlling the transient reduction of the adaptive filter,

-that, if the detector detects presence of transient in a plurality of frequency bands, the filter parameters calculated by the calculating means are arranged to accomplish a total gain to a predetermined level, and

-that, if the detector detects presence of transient in only one frequency band, the filter parameters calculated by the calculating means are arranged to accomplish a total gain to a level determined by the level of the input signal for the current frequency band, -that, if the detector no longer detects presence of transient, the device is arranged to go back to the set of filter parameters, that were used before the detection of the presence of transient, with a predetermined out phasing period.

DESCRIPTION OF THE DRAWINGS

Figure 1 shows a block scheme for detection and reduction of transients in signals in a hearing aid according to the present invention.

Figure 2 shows a block scheme for detection and reduction of transients in signals in a hearing aid according to the present invention, which further e.g. comprises a switch.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention according to a first embodiment relates to a method and device for reduction of transients in signals. The main purpose is to address the drawbacks that have been described earlier. Figure 1 shows a preferred embodiment concerning the present invention which will be described in detail below. The device at least comprises a transient detector 1, a calculating means, which in this embodiment is incorporated in the transient detector 1, means for delaying a signal 2 and an adaptive filter 3. In order for transient detection and the dynamic change of the function of the adaptive filter 3 will be done before the transient reaches a speaker or an actuator a means for delaying a signal 2 is provided. An input signal 4 is arranged to both the means for delaying a signal 2 and the transient detector 1 via parallel connection, which are both connected to the adaptive filter 3, the output signal 5 of which is transient reduced. The incoming control signal 4 to the detector 1 is arranged to be divided into at least two frequency bands, of which the detector 1 is arranged to detect at least one frequency band for the input signal 4 regarding presence of transient with at least one threshold function, which is arranged to indicate presence of transient in at least one frequency band.

Further the detector 1 is arranged to indicate presence of transient if the estimated derivative and/or level of energy or amplitude for the input signal 4 of the detector 1 exceeds predetermined levels. The derivative can be estimated in a number of ways. A way, which is easy to implement, is finite differential quotients. Predetermined levels and values of the derivative can be individually adjusted depending on application. It is within the scope of the invention that these are set depending on e.g. the person who will use the invention. This is particularly the case when the invention is to be used in hearing aids .

The division of the signal 4 into more than one band facilitates a simple form of characterization of the transient. More bands give a better characterization but to the price of increasing complexity and delay through the system, this is why every band for practical purposes not should be narrower than approximately 1 kHz. In the description which follows, the discussion is limited for reasons of simplicity to two bands with the division frequency of 1,5 kHz. This because the maximum amplification in hearing aids usually can be found in the upper band. The investigations presented in "On Transient Noise and Its Reduction in Hearing Aids" work 2 and 4, indicate that a damping independent of frequency is to prefer when the frequency contents of the transients are of broad band character. However, such a type of reduction also implies more degeneration of a contemporaneous speech signal than a corresponding frequency specific transient damping. Every band has thereafter an own threshold function where the value of derivative and level are compared to the threshold values. When both thresholds have been exceeded a transient sound has been detected in the present band whereby the reduction phase is initiated. It is possible to use only one of the thresholds to indicate presence of transient. This however leads to a somewhat lower performance for the invention.

Upon indication of transients in at least one frequency band the detector 1 is arranged to activate a calculating means for calculating a set of filter parameters, which describe the amplification of the frequency bands. When transients are not detected the adaptive filter 3 uses the set of parameters which was found suitable in this specific application. That is, it is first at the detection of a transient the filter parameters are changed. After the duration of the transient the filter parameters return to the setting which was current prior to the transient .

If the detector 1 detects presence of transient in several or all frequency bands are the filter parameters calculated by the calculating means arranged to achieve a total amplification to a predetermined level. If the detector 1 detects presence of transient in only one frequency band, the filter parameters calculated by the calculating means arranged to accomplish a total amplification to a level determined by the level of the input signal for the current frequency band. The detecting can be performed on a continuous basis or with a frequency, which can be variable.

It should be noted that it is fully possible within the scope of the present invention that each frequency band in the detector presents a unique threshold function concerning values of an estimated derivative and/or predetermined level. The values of an estimated derivative and levels can thus be different between the frequency bands. The levels are related to the signal effect of the input signal for respective frequency band. The filter parameters are arranged to change with the zero crossings of the input signal or by windowing e.g. with a time function of the type 1-A1 (1-exp (-t/T) ) , where Al and T are constants. ^λExp' denotes the exponential function. The adaptive filter 3 should be a FIR filter (Finite Impulse Response) . Further a speech detector can be connected. It is preferably based on phonemes characteristics in order to be able to detect transient with an even higher accuracy.

When the detector no longer detects presence of transients the invention is arranged to return to the set of filter parameters, that was used prior to the detection of presence of transient, with a predetermined out phasing period. With out phasing period is intended the time interval under which the transition takes place from transient reduction to the use of the filter parameters that are used when reduction of transients is not performed. The predetermined time period can be variable from zero seconds to 10-100 times the duration of the transient.

A second embodiment, which is shown in figure 2, presents great similarities with figure 1. Only the differences will therefore be described. The device according to this embodiment also comprises a switch 6 and a automatic volume control 7. Essential details are that the automatic volume control 7 is arranged to control the input signal 4 automatically, which also is arranged to the switch 6, which is arranged to connect the output signal 5 of the adaptive filter 3 or the output signal of the automatic volume control to the output signal of the automatic volume control. Further the switch 6 is when the detector 1 does not detect presence of transient, arranged to connect automatically the output signal 4 of the automatic volume control 7 to the control signal 8 of the automatic volume control 7. When the detector 1 detects presence of transient, the switch 6 is arranged to connect the output signal 5 of the adaptive filter 3 to the control signal 8 of the automatic volume control 7 or is arranged to lock the signal to the value prior to the detection of the presence of the transient. The purpose of the switch 6 is to move the inlet point for the input signal of the automatic volume control 7 to a position after the adaptive filter 3, alternatively maintain the state of the control signal 8 to the automatic volume control. This eliminates the drawback that several fast transients will lower the amplification which according to the previous discussion can result in that the succeeding speech signal will be unhearable . The signal delay 2 deals with the drawback of the attack time of the detector 1 admitting a part of the initially very high sound levels which earlier were allowed to pass through the hearing aid with full amplification.

A third embodiment, which is not shown in a figure, comprises beyond what has been presented in the first two embodiments, also at least one microphone, an A/D converter, a D/A converter and a speaker or actuator, where the microphone is connected to the A/D converter, the output signal of which is arranged to be the input signal to both the means for delaying a signal and the transient detector, and the output signal of the adaptive filter is arranged to be the input signal to the D/A converter. The invention according to this example can be used with a plurality of areas, such as telephony (including cellular telephony) , in transmission equipment and in ear protection equipment. Also within the scope of the present invention is a communication device including a device according to claims 1-8, further comprising a microphone, an A/D converter, a D/A converter and a speaker or an actuator, where the microphone is connected to the A/D converter, the output signal of which is arranged to be the input signal to both the means for delaying a signal and the transient detector, and the output signal of the adaptive filter is arranged to be the input signal to the D/A converter. The communication device can be a hearing aid, telephone, cellular phone, transmission equipment, ear protection equipment, telephone switch or the like. In order to increase the use of the invention in situations where transients almost can be seen as a natural component in the sound picture it is within the scope of the invention that if the time interval between consecutive transients, detected by the detector, is shorter than a variably predetermined time, then the reduction of transient is arranged to be inhibited. The variably predetermined time is in this embodiment within the range of 0,1 to 1 seconds.

At implementation of the present invention there is a number of aspects that has to be reflected upon. Realisation of transient reduction filters can principally be accomplished in two different ways. The first relates to use of a permanent filter bank where one can dynamically vary the amplification for respective bands. The other way relates to dynamic changes of a filter by changing its parameters, which works well if the filter is of the type FIR.

If a fast change of transition characteristics will be made (which may be the case at transient reduction) a side effect occurs which can be compared to a scratch on an LP record. This scratch occurs when the signal fast changes its amplitude. This phenomenon can be minimised by synchronising the changes in filter/amplification in connection to zero crossings of the surrounding signals. Further the phenomenon in a speech signal is affected in the way that e.g. an "s" sound can be transformed into an ^Xst" sound. This can lead to that words are misinterpreted why only minor changes should be done to the frequency characteristics. Hereby, a speech detector (Elberling et al, "A method and an apparatus for classification of a mixed speech and noise signal", international patent application 1990) and a phoneme identifier are used to monitor the transient reduction phase at an optimum.

It should be noted that the possibilities to implement the present invention present a great variation. The alternatives range from hardware implementation (analogue) to softwaTe implementation (digital) . Some features of the invention can advantageously be implemented digitally as others may be realized analogously. The spectrum between these extremes are considered to be within the scope of the patent claims.

Claims

PATENT CLAIMS

1. Device for reduction of transients in signals, where t^'he signal is divided into one or more frequency bands, the device at least comprising a transient detector (1), a calculating means, means for delaying a signal (2) and an adaptive filter (3) in which device an input signal (4) is arranged to both the means for delaying a signal (2) and the transient detector (1) via a parallel connection, which are both connected to the adaptive filter (3), the output signal (5) of which is transient reduced, c h a r a c t e r i z e d i n,

-that the detector (1) is arranged to detect at least one frequency band for the input signal (4) regarding presence of transient with at least one threshold function for indicating presence of transient in at least one frequency band, -that the detector (1) is arranged to indicate presence of transient if an estimated derivative and/or energy or amplitude level for the detector (1) input signal (4) exceeds predetermined levels, -that the detector (1), when indicating transient in at least one frequency band, is arranged to activate the calculating means for calculation of a set of filter parameters, describing the gain of the frequency bands, for controlling the transient reduction of the adaptive filter (3) , -that, if the detector (1) detects presence of transient in a plurality of frequency bands, the filter parameters calculated by the calculating means are arranged to accomplish a total gain to a predetermined level, and

-that, if the detector (1) detects presence of transient in only one frequency band, the filter parameters calculated by the calculating means are arranged to accomplish a total gain to a level determined by the level of the input signal (4) for the current frequency band,

-that, if the detector (1) no longer detects presence of transient, the device is arranged to go back to the set of filter parameters, that were used before the detection of the presence of transient, with a predetermined out phasing period.

2. Device according to claim 1, c h a r a c t e r i z e d i n, -that each frequency band in the detector (1) is arranged with a unique threshold function regarding values of estimated derivative and/or predetermined level.

3. Device according to claim 1, c h a r a c t e r i z e d i n,

-that the device further comprises a switch (6) and an automatic volume control (7),

-that the automatic volume control (7) is arranged to control automatically the input signal (4), also being arranged to the switch (6) being arranged to connect the output signal of the adaptive filter (3) or the output signal of the automatic volume control (7) to the control signal (8) of the automatic volume control (7 ) ,

-that the switch (6), when the detector (1) does not indicate presence of transient, is arranged to connect the output signal of the automatic volume control (7) to the control signal of the automatic volume control (7), and

-that the switch (6), when the detector (1) detects presence of transient, is arranged to connect the output signal of the adaptive filter (3) to the control signal of the automatic volume control (7) or is arranged to lock the signal to the value before the detection of the presence of transient.

4. Device according to claim 1, c h a r a c t e r i z e d i n,

-that the predetermined levels are related to the signal effect of the input signal in respective frequency bands.

5. Device according to claim 1, c h a r a c t e r i z e d ! n,

-that the filter parameters are arranged to be altered with the zero crossings of the input signal, or by windowing e.g. with a time function of the type

1-A1 (l-exp(-t/T) ) ,

where Al and T are constants,

6. Device according to any of the claims 1 to 5, c h a r a c t e r i z e d i n,

-that the adaptive filter (3) is an FIR filter.

7. Device according to claim 1, c h a r a c t e r i z e d i n,

-that the device further comprises a speech detector, which is preferably based on phonemes characteristics.

8. Device according to claim 1, c h a r a c t e r i z e d i n,

-that if the time interval between consecutive transients, detected by the detector (1), is shorter than a variably predetermined time, the reduction of the transient is arranged to be inhibited.

9. Communication device comprising a device according to claims 1 to 8, further comprising a microphone, an A/D converter, a D/A converter and a speaker or an actuator, where the microphone is connected to the A/D converter, the output signal of which is arranged to be the input signal to both the means for delaying a signal (2) and the transient detector (1), and the output signal of the adaptive filter (3) is arranged to be the input signal to the D/A converter.

10. Communication device according to claim 9, c h a r a c t e r i z e d ! n,

-that the communication device is a hearing aid, telephone, cellular telephone, transmission equipment, ear protection equipment, telephone switch or the like.

11. Method for reduction of transients in signals in a device, where the signal is divided into one or more frequency bands, the device at least comprising a transient detector (1), a calculating means, means for delaying a signal (2) and an adaptive filter (3) in which method an input signal (4) is arranged to both the means for delaying a signal (2) and the transient detector (1) via a parallel connection, which are both connected to the adaptive filter (3), the output signal (4) of which is transient reduced, c h a r a c t e r i z e d i n,

-that the detector (1) detects at least one frequency band for the input signal (4) regarding presence of transient with at least a threshold function for indicating presence of transient in at least one frequency band,

-that the detector (1) indicates presence of transient if an estimated derivative and/or energy or amplitude level for the detector (1) input signal (4) exceeds predetermined levels, -that the detector (1), when indicating transient in at least one frequency band, activates a calculating means for calculation of a set of filter parameters, describing the gain of the frequency bands, for controlling the transient reduction of the adaptive filter (3), -that, if the detector (1) detects presence of transient in a plurality of frequency bands, the filter parameters calculated by the calculating means accomplish a total gain to a predetermined level, and

-that, if the detector (1) detects presence of transient in only one frequency band, the filter parameters calculated by the calculating means accomplish a total gain to a level determined by the level of the input signal (4) for the current frequency band,

-that, if the detector (1) no longer detects presence of transient, the device goes back to the set of filter parameters, that were used before the detection of the presence of transient, with a predetermined out phasing period.

12 . Method according to claim 11 , c h a r a c t e r i z e d i n, -that the device further comprises a switch (6) and an automatic volume control (7),

-that the automatic volume control (7) automatically controls the input signal, that connects the output signal of the adaptive filter (3) or the output signal of the automatic volume control (7) to the control signal (8) of the automatic volume control (7),

-that the switch (╬▓), when the detector (1) does not indicate presence of transient, connects the output signal of the automatic volume control (7) to the control signal (8) of the automatic volume control (7), and

-that the switch (6), when the detector (1) detects presence of transient, connects the output signal (5) of the adaptive filter (3) to the control signal (8) of the automatic volume control (7) or locks the signal to the value before the detection of the presence of transient.